The present invention relates to a knock suppression apparatus and method for a multi-cylinder internal combustion engine which can detect knocking in each of the engine cylinders and control engine operating parameters in a direction to suppress the knocking therein. More particularly, it relates to a knock suppression apparatus and method which can selectively prevent such knock suppression control from being reflected or performed on a cylinder for which knock determination is determined to be erroneous due to the influences of noise.
In general, internal combustion engines such as automotive gasoline engines have a plurality of cylinders for which ignition timing, the order of fuel injection, etc, are required to be properly controlled in accordance with the engine operating conditions such as the engine load for the purpose of generating output torque in an effective manner. To this end, the ignition timing for example is controlled to advance as the engine rotational speed increases, but if it is advanced to an excessive extent, premature or abnormal vibrations known as knocking due to abnormal combustion will result, giving rise a fear that the engine may be thereby damaged. In order to prevent, such an undesirable situation, when abnormal vibrations of the engine are detected upon ignition, it is general practice to control engine operating parameters in a direction to suppress the knocking, for example, to retard ignition timing in accordance with the engine vibrations.
FIG. 4 shows a typical known knock suppression apparatus for an internal combustion engine. The apparatus includes a knock sensor 1 in the form of a piezoelectric element and the like installed on the engine for sensing knocking therein, e.g., for sensing vibrations of the engine due to knocking and generating a corresponding electrical output signal A.
An interface circuit, generally designated by reference numeral 2, receives the output signal A from the knock sensor 1 and generates a peak level P and an average level A* during each ignition cycle. The interface circuit 2 includes a band-pass filter 21 for allowing the passage of a specific frequency band (e.g., around 7 KHz) attributable to knocking, a peak-hold circuit 22 for holding the peak level P of the output signal A from the filter 21 at a prescribed reference crank position for each cylinder which corresponds to the ignition timing thereof, and an averaging circuit 23 for averaging the output signal A from the filter 21 and generating a corresponding average level A*.
A first A/D converter 31 converts the peak level P in analog form from the peak-hold circuit 22 into a digital value V.sub.P. A second A/D converter 32 converts the average level A* in analog form from the averaging circuit 23 into a digital value V.sub.A.
An electronic control unit 4 (hereinafter simply referred to as an ECU), generally designated by reference numeral 4, controls the ignition timing or instant for a cylinder based on the A/D converted peak level V.sub.P and the A/D converted average level V.sub.A, and generates a mask singal M to the peak-hold circuit 22. The ECU 4 includes a peak threshold calculator 41 for amplifying the digitized average level V.sub.A and adding an offset value to it to provide a threshold level or a peak threshold V.sub.TH, a substracter 42 for making a comparison between the peak level V.sub.P and the peak threshold V.sub.TH to provide a deviation level .DELTA.V, and a retard angle controller 43 for generating a retarded control angle .theta..sub.R for ignition retarding control based on the deviation level .DELTA.V.
The retard angle controller 43 comprises a retard angle calculator 44 for calculating an amount or angle of retardation .DELTA..theta..sub.R for each ignition on the basis of the deviation level .DELTA.V, and a retard angle adder 45 for successively summing the amount or angle of retardation .DELTA..theta..sub.R thus obtained to successively provide a retarded ignition angle .theta..sub.R.
The subtracter 42 acts to generate a deviation level in the form of a knock determination signal .DELTA.V based on the peak level V.sub.P corresponding to the output signal A from the knock sensor 1. Also, the retard angle controller 43 acts to control engine operating parameters in a direction to suppress knocking on the basis of the deviation level or the knock determination signal .DELTA.V.
Here, it to be noted that the mask signal M contains a series of square pulses, which are generated by the ECU 4 at prescribed intervals, i.e., at a prescribed reference crank position of each cylinder during the rotation of the engine. Thus, for example, each square pulse rises at a first reference crank position of 75 degrees before top dead center (BTDC) for each cylinder and falls at a second reference crank position of 5 degrees BTDC, as clearly shown at M in FIG. 2. Accordingly, the peak-hold circuit 22 is disabled by the mask signal M in an angular range from the first reference crank position (e.g., 75 degrees BTDC) to the second reference crank position (e.g., 5 degrees BTDC) whereas it is enabled to hold the peak level P during a time from the second reference crank angle to the first reference crank angle.
The operation of the above-mentioned known knock suppression apparatus will now be described in detail.
In general, each cylinder is ignited at a reference crank position of about 5 degrees before TDC, so explosive combustion of a mixture therein takes place at around a crank angle position ranging from 10 to 60 degrees after TDC. As a result, knocking due to abnormal combustion would generally occur at such a crank angle position (around 10-60 degrees after TDC). Accordingly, if knocking takes place in a cylinder, the output signal A from the knock sensor 1 will have a wave form whose amplitude periodically becomes the greatest at around 10-60 degrees after each TDC, as clearly seen from FIG. 2.
The filter 21 in the interface circuit 2 allows the passage of a knock component contained in the output signal A from the knock sensor 1, which results from vibrations of a cylinder due to knocking. The peak-hold circuit 22 outputs a peak level of each output signal A. The averaging circuit 23 generates an average level A* corresponding to a background level of the output signal A. The ECU 4 generates a mask signal M to the peak-hold circuit 22 so that it can receive the peak wave of the knock sensor output A in an effective manner.
The ECU 4 receives the analog-to-digital converted peak level V.sub.P and the average level V.sub.A of which the latter is then properly amplified by the threshold calculator 41 and offset added to the background level of the knock sensor output A to provide a peak threshold V.sub.TH as a knock detection reference.
The subtracter 42 determines the occurrence of knocking if the peak level V.sub.P exceeds the peak threshold V.sub.TH, and outputs a deviation level .DELTA.V (.DELTA.V=V.sub.P -V.sub.TH) as a high level to the retard angle controller 43.
The retard angle calculator 44 in the retard angle controller 43 calculates an amount or angle of retardation .DELTA..theta..sub.R for each ignition required for knock suppression based on the deviation level .DELTA.V. Based on the amount of retardation .DELTA..theta..sub.R, the retard angle adder 45 ouputs a retarded control angle .theta..sub.R in order to retard an ignition instant in a direction to suppress knocking. In this connection, the retarded control angle .theta..sub.R is expressed as follows: EQU .theta..sub.R =.theta..sub.R *+.DELTA..theta..sub.R
where .theta..sub.R * is the last retarded control angle. With this, an ignition instant of a cylinder to be controlled is modified in an ignition retarding direction so that there will no longer be any knocking in the cylinder.
In this case, however, cylinders are subject to various physical vibrations during the operation thereof, and thus the output signal A from the knock sensor 1 usually contains resultant noise. For example, intake and exhaust valves in each cylinder are properly driven to open and close in an alternative manner so as to supply an air/fuel mixture to a cylinder, compress the mixture therein, exhaust combusted gases generated after explosion, and the like, so that impactive forces developed upon opening and closing of these valves at the time of ignition will sometimes cause vibration of a cylinder, which can be mistakenly sensed as knocking by a knock sensor 1. Generally, such cylinder vibrations due to noise continue for a certain period of time. In addition, even if the level of vibrations due to noise is less than that due to knocking, it can exceed the peak threshold V.sub.TH.
In this case, if ignition retarding control is performed based on an incorrect determination that noise is mistaken as knocking, an ignition instant is cumulatively controlled in an ignition retarding direction each time knocking is mistakenly sensed, thus greatly impairing cylinder control efficiency. In particular, provided that a knock sensor 1 is mounted on one cylinder alone, incorrect knock detection results in degradation in the overall control efficiency for all the cylinders, giving rise to a big problem from the point of view of effective engine control.
Thus, with the known knock suppression apparatus and method as described above in which an ignition instant of each cylinder is controlled in an ignition retarding direction on the basis of the result of a comparison between the peak level V.sub.P and the peak threshold V.sub.TH, it is unable to perform accurate knock determination in cases where the knock sensor output A contains a noise component which is greater than the peak threshold V.sub.TH. In this case, the noise component is mistakenly detected as knocking, so the ignition instant is continuously controlled to retard, thus considerably reducing engine control efficiency.